Wireless communication system and wireless communication method

ABSTRACT

Provided is a wireless communication system in which, based on a random access scheme, a transmission opportunity (TXOP) acquired by a wireless access point is granted to a wireless station when performing uplink data transmission from the wireless station to the wireless access point and the TXOP acquired by the wireless station is granted to the wireless access point when performing downlink data transmission from the wireless access point to the wireless station, the system includes a scheduling unit for TXOP granting determining one or more wireless stations having a quality of an uplink data transmission lower than a prescribed value as TXOP responders to which the wireless access point grants the TXOP, and determining one or more wireless stations having a quality of a downlink data transmission lower than the prescribed value as TXOP holders which grant the TXOP to the wireless access point.

TECHNICAL FIELD

The present invention relates to granting of a transmission opportunity(TXOP) when performing downlink data transmission from a wireless accesspoint (AP) to a wireless station (STA) or granting of the TXOP whenperforming uplink data transmission from the STA to the AP, in awireless communication system and a wireless communication method inwhich communication is performed based on a random access scheme such asCarrier Sense Multiple Access/Collision Avoidance (CSMA/CA).

Particularly, the present invention relates to processing that selects aTXOP holder that is a STA which grants a TXOP to an AP in order toperform downlink data transmission. Furthermore, the present inventionrelates to processing that selects a TXOP responder that is a STA towhich an AP grants a TXOP, in order to perform uplink data transmission.

BACKGROUND ART

Wireless Local Area Network (LAN) systems in a standard IEEE 802.11 havebeen improved in throughput year after year, and have come intowidespread use as one example of main wireless access (Non-PatentDocument 1). Because the wireless LAN system can be used in anunlicensed band that is a frequency band which does not require alicense, various types of wireless stations have come into widespreaduse. Particularly, the spread of smartphones remarkably increases anopportunity to use the wireless LAN system.

A 2.4 GHz band, a 5 GHz band, and a 60 GHz band are assigned asunlicensed bands that are frequency bands which are used by the wirelessLAN system. In microbands, a 2.4 GHz band and a 5 GHz band, a band thatcan be used by the wireless LAN system in Japan is approximately 500MHz. Furthermore, a band that is equal to or greater than 500 MHz isassigned as a band that is possibly used abroad by the wireless LANsystem. On the other hand, because wireless communication in anunlicensed band is performed based on a random access scheme such asCSMA/CA, a hidden terminal problem is a great obstacle. Flow controlthat uses request-to-send (RTS) and clear-to-send (CTS) is proposed forthe purpose of solving the hidden terminal problem.

At this point, as illustrated in FIG. 8, two wireless LANs, a basicservice set (BSS) a and a BSS b, are present in a communication areawhere an AP 10 and a STA 20-1 to a STA 20-3 communicate with each other,using an unlicensed band. The AP 10 and the STA 20-1 are present inpositions where wireless signals of an AP a and a STA a on one wirelessLAN, the BSS a, can be detected. The STA 20-1 and the STA 20-2 arepresent in positions where wireless signals of an AP b and a STA b onthe other wireless LAN, the BSS b, can be detected. The STA 20-3 ispresent in a position where communication between the BSS a and the BSSb cannot be detected.

First, in the uplink data transmission (transmission from the STA to theAP) that is illustrated in (a) of FIG. 8, a case where the STA 20-1acquires a TXOP and transmits a data frame does not pose any problem.However, when the STA 20-3 is set to acquire the TXOP and transmit thedata frame, a collision occurs in the AP 10 due to communication overthe BSS a that is a hidden terminal from the standpoint of the STA 20-3,or a failure in the uplink data transmission occurs because of a NetworkAllocation Vector that is set for the AP 10. In this case, the AP 10acquires the TXOP, and grants the acquired TXOP to the STA 20-3. Thus,an influence of a hidden terminal that is viewed from the STA 20-3 isreduced and throughput of the uplink from the STA 20-3 to the AP 10 canbe improved (Non-Patent Document 3).

Next, the downlink data transmission (transmission from the AP to theSTA) in (b) of FIG. 8 will be considered. In communication that is basedon the RTS/CTS, before the AP 10 transmits user data to the STA 20-1, anRTC frame is transmitted when a wireless signal over a wireless channel,which is transmitted by the AP 10, cannot be detected, the STA 20-1transmits a CTS frame according to the RTS frame, and the AP a and theSTA a on the BSS a set the network allocation vector (NAV) according tothe RTS frame or the CTS frame. Furthermore, the AP b and the STA b onthe BSS b set the NAV according to the CTS frame that is transmitted bythe STA 20-1. Accordingly, the BSS a stops the communication, and alongwith this, the BSS b that is a hidden terminal from the standpoint ofthe AP 10 can stop the communication, and can perform transmission tothe STA 20-1 over a wireless channel over which the AP 10 acquires theTXOP.

In this manner, when it comes to exchange of the RTS frame and the CTSframe, even if the wireless signal over the BSS b, which cannot bedetected from the AP in the STA, is present, the STA sets the NAV usingthe CTS frame, according to the RTS frame from the AP. Thus, it ispossible that the hidden terminal problem is solved and that a decreasein throughput is prevented.

Incidentally, in 3GPP, a standardization organization, the discussion ona cellular system that uses an unlicensed band for a wireless LAN hasbegun (Non-Patent Document 2) as well, and attention has been focused ona method of utilizing the unlicensed band. That is, in the cellularsystem, the utilization of a frequency band (the unlicensed band) forthe wireless LAN in compliance with specifications for the unlicensedband has been discussed as well. The description as the wireless LAN isprovided here, but any communication system in which communication isperformed with the random access in the unlicensed band may beavailable.

For example, in a situation that is illustrated in (b) of FIG. 8, when adata frame from the AP 10 to the STA 20-1 is transmitted, an environmentwhere wireless resources are insufficient for the wireless signal of theterminal in the BSS b that is the hidden terminal from the AP 10 (anenvironment where the wireless signal is always transmitted) isconsidered. In this case, a frame that is transmitted from the AP 10experiences a collision, and thus is not received in the STA 20-1, ordue to the NAV that is set by the BSS b, the STA-20 cannot transmit theCTS frame in response to the RTS frame that is transmitted from the AP10. As a result, a problem occurs in that although control which isbased on the RTS and CTS is performed, throughput decreases. To copewith this problem, the STA 20-1 sets the NAV or a transmissionprohibition section that has the same function as the NAV, for the BSS bthat is the hidden terminal from the AP 10 and thus acquires the TXOP.Then, the STA 20-1 grants the acquired TXOP to the AP 10, and transmitsthe data frame using the TXOP that is granted by the AP 10. Thus, aninfluence of the hidden terminal in the AP 10 is reduced and throughputof downlink from the AP 10 to the STA 20-1 can be improved (Non-PatentDocument 3).

PRIOR ART DOCUMENTS Non-Patent Documents

Non-Patent Document 1: IEEE Std 802.11ac(TM)-2013, IEEE Standard forInformation technology—Telecommunications and information exchangebetween systems Local and metropolitan area networks—Specificrequirements, Part 11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) Specifications, December 2013

Non-Patent Document 2: RP-140057, “On the primacy of licensed spectrumin relation to the proposal of using LTE for a licensed-assisted accessto unlicensed spectrum”, 3GPP TSG-RAN #63, March 2014

Non-Patent Document 3: R. Kudo, B. A. H. S. Abeysekera, Y. Takatori, T.Ichikawa, M. Mizoguchi, H. Yasuda, A. Yamada, Y. Okumura, “Channelaccess acquisition mechanism coupled with cellular network forunlicensed spectrum”, in Proc., VTC2015-Spring, May 2015

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, there is a problem in that the unfairness in throughput betweenthe STAs is caused to occur depending on how to make a selection, thatis, how to select the STA to which the AP grants the TXOP in order toperform the uplink data transmission, or how to select the STA thatgrants the TXOP to the AP in order to perform the downlink datatransmission. For example, although like the STA 20-3 that isillustrated in (a) of FIG. 8, the STA itself acquires the TXOP in orderto perform the transmission, if throughput of the uplink datatransmission decreases remarkably due to the communication over the BSSa that is in a hidden-terminal relation, it is effective to grant theTXOP that is acquired by the AP 10 to the STA 20-3. On the other hand,as is the case with the STA 20-2 that is in a position of the hiddenterminal from the BSS a, although the TXOP that is acquired by the AP 10is uniformly granted to an arbitrary STA, this does not necessarilycontribute to an improvement in throughput.

Furthermore, if the TXOP that is to be granted to the AP 10 in order toperform the downlink data transmission is set to be acquired by allSTAs, the number of nodes that participate in random access on thedownlink increases dramatically and system throughput of the downlink isgreatly improved. However, a problem occurs as well in that one STAoccupies many wireless resources in contrast to the BSS in the vicinityand the unfairness between the BSSs is caused to occur. In this case, ifthe number of STAs that are set to acquire the TXOP is configured to beas small as possible, an influence that is exerted to the communicationover the BSS in the vicinity can be decreased, but there is a need tosuitably select the STA in which the throughput is greatly improved bygranting the TXOP to the AP 10.

A preposition of the present invention is to provide a wirelesscommunication system and a wireless communication method that arecapable of efficiently performing selection of an STA to which an APgrants a TXOP in order to perform uplink data transmission and selectionof the STA that grants the TXOP to the AP in order to perform downlinkdata transmission in the wireless communication system that is based onrandom access.

Means for Solving the Problems

According to a first invention, there is provided a wirelesscommunication system in which, based on a random access scheme, a TXOPacquired by a wireless access point is granted to a wireless stationwhen performing uplink data transmission from the wireless station tothe wireless access point and the TXOP acquired by the wireless stationis granted to the wireless access point when performing downlink datatransmission from the wireless access point to the wireless station, thesystem including a scheduling unit for TXOP granting determining one ormore wireless stations having a quality of the uplink data transmissionlower than a prescribed value as TXOP responders to which the wirelessaccess point grants the TXOP, and determining one or more wirelessstations having a quality of the downlink data transmission lower thanthe prescribed value as TXOP holders which grant the TXOP to thewireless access point.

In the wireless communication system according to the first invention,the scheduling unit for TXOP granting may be configured to determine oneor more of the TXOP responders having the communication quality of theuplink which is indicated by a parameter, which indicates the quality ofthe uplink data transmission, lower than the prescribed value, and totransmit a notification signal representing information on thedetermination to the wireless access point; the wireless access pointmay include a unit transmitting a TXOP granting signal indicating atiming to perform the uplink data transmission to the TXOP respondersafter acquiring the information on the determination of the TXOPresponders by the notification signal and acquiring the TXOP using therandom access scheme; and the wireless station may include a unittransmitting a data frame on the uplink at the indicated timing whenreceived the TXOP granting signal.

In the wireless communication system according to the first invention,the scheduling unit for TXOP granting may be configured to use, as theparameter indicating the quality of the uplink data transmission, atleast one of a value indicating degradation in the quality of the uplinkdata transmission in contrast to the quality of the downlink datatransmission, a frame error rate of the uplink, a TXOP acquisition rateof the uplink, a ratio of throughput to a QoS condition or a trafficvolume demand in the uplink, and a frame retransmission rate of theuplink.

In the wireless communication system according to the first invention,the scheduling unit for TXOP granting may be configured to collectuplink traffic information relating to a traffic volume or atransmission-awaiting packet generated by the wireless station, anddetermine as the TXOP responders when the quality of the uplink datatransmission lower than the prescribed value and a demand for uplinktraffic in the wireless station is higher than the prescribed value.

In the wireless communication system according to the first invention,the scheduling unit for TXOP granting may be configured to determine oneor more of the TXOP holders having the communication quality of thedownlink which is indicated by a parameter, which indicates the qualityof the downlink data transmission, lower than the prescribed value, andto transmit a notification signal representing information on thedetermination to the wireless station; the wireless station may includea unit transmitting a TXOP granting signal indicating a timing toperform the downlink data transmission to the wireless access pointafter recognizing as the TXOP holders by the notification signal andacquiring the TXOP using the random access scheme; and the wirelessaccess point may include a unit transmitting a data frame on thedownlink at the indicated timing when received the TXOP granting signal.

In the wireless communication system according to the first invention,the scheduling unit for TXOP granting may be configured to use, as theparameter indicating the quality of the downlink data transmission, atleast one of a value indicating degradation in the quality of thedownlink data transmission in contrast to the quality of uplink datatransmission, a frame error rate of the downlink, a ratio of throughputto a QoS condition or a traffic volume demand in the downlink, and aframe retransmission rate of the downlink.

In the wireless communication system according to the first invention,the scheduling unit for TXOP granting may be configured to collectdownlink traffic information relating to a traffic volume or atransmission-awaiting packet from the wireless access point to thewireless station, and to determine as the TXOP responders when thequality of the downlink data transmission is lower than the prescribedvalue and a demand for downlink traffic in the wireless access point ishigher than the prescribed value.

According to a second embodiment, there is provided a wirelesscommunication method in which, based on a random access scheme, a TXOPacquired by a wireless access point is granted to a wireless stationwhen performing uplink data transmission from the wireless station tothe wireless access point, the method including a step causing ascheduling unit for TXOP granting, which monitors a quality of theuplink data transmission, to determine one or more wireless stationshaving a parameter, which indicates the quality of the uplink datatransmission, lower than a prescribed value as TXOP responders to whichthe wireless access point grants the TXOP, and to transmit anotification signal representing information on the determination to thewireless access point; a step causing the wireless access point totransmit a TXOP granting signal indicating a timing to perform theuplink data transmission to the TXOP responders after causing to acquirethe information on the determination of the TXOP responders by thenotification signal and to acquire the TXOP using the random accessscheme; and a step of causing the wireless station to transmit a dataframe on the uplink at the indicated timing when received the TXOPgranting signal.

According to a third invention, there is provided a wirelesscommunication method in which, based on a random access scheme, a TXOPacquired by a wireless station is granted to a wireless access pointwhen performing downlink data transmission from the wireless accesspoint to the wireless station, the method including a step causing ascheduling unit for TXOP granting, which monitors a quality of thedownlink data transmission, to determine one or more wireless stationshaving a parameter, which indicates the quality of the downlink datatransmission, lower than a prescribed value as TXOP holders which grantthe TXOP to the wireless access point, and to transmit a notificationsignal representing information on the determination to the wirelessstation; a step causing the wireless station to transmit a TXOP grantingsignal indicating a timing to perform the downlink data transmission tothe wireless access point after causing to recognize as the TXOP holdersby the notification signal and to acquire the TXOP using the randomaccess scheme; and a step causing the wireless access point to transmita data frame on the downlink at the indicated timing when received theTXOP granting signal.

Advantageous Effects of the Invention

According to the present invention, a wireless station in which userexperience decreases because low quality of uplink data transmission canbe selected as a TXOP responder, based on statistical information or anempirical assumption, and a TXOP that is acquired by an AP can bepreferentially granted to the TXOP. Thus, throughput in the uplink datatransmission can be ensured in terms of fairness and can be improved.

According to the present invention, the wireless station in which theuser experience decreases because low quality of downlink datatransmission can be selected as a TXOP holder, based on the statisticalinformation or the empirical assumption, and a STA grants the TXOP tothe AP. Thus, throughput in the downlink data transmission can beensured in terms of fairness and can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating an example of a configuration of awireless communication system.

FIG. 2 is a diagram illustrating an example of uplink data transmissionthat results from an AP granting a TXOP.

FIG. 3 is a diagram illustrating an example of a configuration of an AP10.

FIG. 4 is a diagram illustrating an example of a configuration of a STA20.

FIG. 5 is a diagram illustrating an example of a configuration of ascheduling unit 30 for TXOP granting.

FIG. 6 is a diagram illustrating an example of a processing procedurefor the uplink data transmission that results from the AP granting theTXOP.

FIG. 7 is a diagram illustrating an example of a processing procedurefor downlink data transmission that results from a STA granting theTXOP.

FIG. 8 is a diagram illustrating an example of a configuration of awireless communication system in the related art.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 illustrates an example of a configuration of a wirelesscommunication system according to the present invention. (a) of FIG. 1illustrates uplink data transmission that results from an AP granting aTXOP, and (b) of FIG. 1 illustrates downlink data transmission thatresults from a STA granting the TXOP.

In (a) and (b) of FIG. 1, an AP 10 and a STA 20-1 to a STA 20-3communicate with each other, and two wireless LANs, a BSS a and a BSS bare present in the vicinity of the AP 10 and the STA 20-1 to STA 20-3.The AP 10 and the STA 20-1 are present in positions where wirelesssignals of an AP a and a STA a on one wireless LAN, the BSS a, can bedetected. The STA 20-1 and the STA 20-2 are present in positions wherewireless signals of an AP b and a STA b on the other wireless LAN, theBSS b, can be detected. The STA 20-3 is present in a position wherecommunication between the BSS a and the BSS b cannot be detected. Abasic configuration is the same as illustrated in FIG. 8, and the AP 10is connected to a network 100.

The AP 10 has a function of granting the acquired TXOP to one or moreSTAs and thus causing the one or more STAs to perform the uplink datatransmission (uplink TXOP granting). Furthermore, the STA 20 has afunction of granting the acquired TXOP to an AP and thus causing the APto perform the downlink data transmission (downlink TXOP granting).

The present invention has a feature of including a scheduling unit 30for TXOP granting that performs processing which selects a STA to whichthe AP 10 grants the TXOP for the uplink data transmission from the STA20-1 to the STA 20-3 to the AP 10 and processing which selects a STAthat grants the TXOP to the AP 10 from the downlink data transmissionfrom the AP 10 to the STA 201 to the STA 20-3. The scheduling unit 30for TXOP granting may be included within the AP 10 and may be includedon the network 100 to which the AP 10 makes a connection.

FIG. 2 illustrates an example of the uplink data transmission thatresults from the AP granting the TXOP. (a) of FIG. 2 illustrates asequence for granting the TXOP acquired by the AP to the STA if the APhas downlink data. (b) of FIG. 2 illustrates a sequence for granting theTXOP acquired by the AP to the STA if the AP does not have the downlinkdata. It is noted that the same is true for a sequence for the downlinkdata transmission that results from the STA granting the TXOP, and thatthe AP and the STA may have a relation as opposed to that which isillustrated in FIG. 2.

Under the control of the scheduling unit 30 for TXOP granting, the APgrants the TXOP to a designated STA and the designated STA is caused toperform the uplink data transmission. The AP transmits an RTC frameafter acquiring the TXOP, and receives a CTS frame that is transmittedfrom the corresponding STA. As illustrated in (a) of FIG. 2, if the APhas downlink data, the AP transmits TXOP granting signal to the STAafter transmitting the data to the STA. Furthermore, as illustrated in(b) of FIG. 2, if the AP does not have the downlink data, the TXOPgranting signal is transmitted to the STA after the CTS is received.With the reception of the TXOP granting signal, the STA transmits uplinkdata based on the TXOP that is granted by the AP. At this point, withthe RTS that is transmitted by the AP or the CTS that is transmitted bythe STA, along with the time at which the uplink data transmission thatresults from granting TXOP is performed, an NAV is set for neighboringnodes A and B.

An important point here is a function of the scheduling unit 30 for TXOPgranting that selects a STA to which the AP grants the TXOP with theuplink data transmission, or a STA that grants the TXOP to the AP withthe downlink TXOP, considering throughput of each STA. A processingprocedure for granting the TXOP for uplink and the TXOP for downlinkwill be described below along with examples of configurations of the AP10 and the STA 20, and the scheduling unit 30 for TXOP granting.

FIG. 3 illustrates the example of the AP 10.

In FIG. 3, the AP 10 is configured with an antenna 11, a transmissionand reception unit 12, a transmission preparation unit 13, a networkcommunication unit 14, and a TXOP granting signal generating unit 15.When user data that is transmitted on the downlink from an externalnetwork through the network communication unit 14 is input, the AP 10stores the received user data in a storage unit of the transmissionpreparation unit 13, and the transmission and reception unit 12 acquiresthe TXOP with random access control, performs RTS/CTS, and then builds adata frame for transmission through the antenna 11. At this point, theAP can have a function of performing transmission to a plurality of STAsusing orthogonal frequency domain multiplexing access (OFDMA) or spatialmultiplexing, and a plurality of STAs that are transmission targets maybe available.

On the other hand, if the AP 10 grants the TXOP to the STA 20 in orderto receive the user data on the uplink, the TXOP granting signalgenerating unit 15 performs its function. When acquiring a TXOP grantschedule from the scheduling unit 30 for TXOP granting that will bedescribed above, the TXOP granting signal generating unit 15 notifiesthe transmission preparation unit 13 that the TXOP is granted to the STAwhich is designated with the TXOP grant schedule. If downlink data ispresent, the transmission preparation unit 13 acquires the TXOP,transmits the downlink data, and then transmits the TXOP granting signalfrom the transmission and reception unit 12 ((a) of FIG. 2), but, if thedownlink data is not present, acquires the TXOP and then transmits theTXOP granting signal directly from the transmission and reception unit12 ((b) of FIG. 2).

FIG. 4 illustrates the example of the configuration of the STA 20.

In FIG. 4, the STA 20 is configured with an antenna 21, a transmissionand reception unit 22, a transmission preparation unit 23, a networkcommunication unit 24, and a TXOP granting signal generating unit 25.When the STA 20 stores user data from the network communication unit 24,which is transmitted on the uplink, in a storage unit of thetransmission preparation unit 23, the transmission and reception unit 22acquires the TXOP with the random access control, performs the RTS/CTScontrol, and then builds a data frame for transmission from the antenna21. The network communication unit 24 possibly performs communicationwith a functional block within the STA, and can input and output asignal from and to other functional blocks within the STA, such asblocks for a portable communication network, wired communication,optical communication, and infrared communication.

Furthermore, if the STA 20 receives a signal on the downlink, the signalthat is received in the antenna 21 is decoded in the transmission andreception unit 22, and the decoded code is output to the networkcommunication unit 24 through the transmission preparation unit 23. Ifthe TXOP granting signal that is transmitted from the AP 10 is includedin the decoded signal, the uplink data that waits for transmission inthe transmission preparation unit 23 is transmitted from thetransmission and reception unit 22 on the uplink at a designated timing.

On the other hand, if the STA 20 grants the TXOP to the AP 10 in orderto receive the signal on the downlink, the TXOP granting signalgenerating unit 25 performs its function. When acquiring the TXOP grantschedule from the scheduling unit 30 for TXOP granting, which will bedescribed above, through the AP 10, the TXOP granting signal generatingunit 25 notifies the transmission preparation unit 23 that the STA towhich the TXOP granting signal generating unit 25 belongs acquires theTXOP with the TXOP grant schedule and grants the acquired TXOP to the AP10. If uplink data is present, the transmission preparation unit 23acquires the TXOP, transmits the uplink data, and then transmits theTXOP granting signal from the transmission and reception unit 22, but,if the uplink data is not present, acquires the TXOP and then transmitsthe TXOP granting signal directly from the transmission and receptionunit 22.

FIG. 5 illustrates an example of the configuration of the schedulingunit 30 for TXOP granting.

In FIG. 5, the scheduling unit 30 for TXOP granting is configured with anetwork communication unit 31, a communication quality determining unit32, and the schedule fixing unit 33 for TXOP granting. The communicationquality determining unit 32 collects information relating tocommunication quality of the uplink or the downlink, or both. As theinformation to the communication quality, there is a frame error rate ofthe uplink/downlink or a TXOP acquisition rate of the uplink/downlink, aratio of throughput to a quality of service (QoS) condition or a trafficvolume demand in the uplink/downlink, or a frame retransmission of theuplink/downlink. Moreover, the communication quality determining unit 32collects information on a function of the STA, contract information ofan owner of the STA, positional information on the STA, information ontraffic that occurs to the STA or on a packet awaiting transmission, andinformation on an application that is used by the STA, and can use thesepieces of information for determining the TXOP grant schedule.

Based on communication quality information of the communication qualitydetermining unit 32, the schedule fixing unit 33 for TXOP grantingselects a STA to which the AP grants the TXOP or a STA that grants theTXOP to the AP, or a STA to which the TXOP is granted and which grantsthe TXOP, determines schedule information for TXOP granting that is setfor each of the selected STA, and notifies the AP or the STA, or both ofthe determined schedule information through the network communicationunit 31.

When the scheduling unit 30 for TXOP granting notifies the AP 10 of theschedule information for TXOP granting, a channel for a network thatconnects between the scheduling unit 30 for TXOP granting and the AP 10can be used. When the scheduling unit 30 for TXOP granting notifies theSTA 20 of the schedule information for TXOP granting, the schedulingunit 30 for TXOP granting may notify the STA 20 of whether to use thechannel for the network that connects between the scheduling unit 30 forTXOP granting and the AP 10 or a wireless channel between the AP 10 andthe STA 20, using a channel on which an external communication apparatusserves as an intermediary. However, if the scheduling unit 30 for TXOPgranting is included in the AP 10, the schedule information for TXOPgranting can be input and output directly within a circuit, withoutusing the channel for the network between the scheduling unit 30 forTXOP granting and the AP 10.

(Uplink TXOP Granting)

Uplink TXOP granting will be described in detail below.

If the STA itself acquires the TXOP in the uplink, a probability that asignal of the neighboring BSS will be always detected and thus the TXOPwill be able to be acquired is considerably low, a collision with asignal that is transmitted from a node which is a hidden terminal fromthe standpoint of the STA in the AP on the reception side is experiencedand thus the communication is not successful, and so forth. Thus, thescheduling unit 30 for TXOP granting selects a STA that hascommunication quality of the uplink that results from the acquisition ofthe TXOP, which causes user experience to decrease, and sets theselected STA to be a “TXOP responder” to which the AP 10 grants theTXOP.

In an example in (a) of FIG. 1, when the STA 20-1 is set to acquire theTXOP on the uplink, a probability that the TXOP will be acquired by thecommunication over the BSS b decreases. Furthermore, when the STA 20-3is set to acquire the TXOP on the uplink, the communication over the BSSa that is in a hidden-terminal relation cannot be detected, thecollision with the signal occurs in the AP 10 and thus a probabilitythat the communication will fail increases. The TXOP that is acquired bythe AP 10 is preferentially granted to the STA 20-1 and the STA 20-3 asin such cases, and thus the user experience can be improved.

FIG. 6 illustrates an example of a processing procedure for the uplinkthat results from the AP granting TXOP.

In FIG. 6, the communication quality determining unit 32 of thescheduling unit 30 for TXOP granting collects communication qualityinformation on the uplink or the uplink/downlink (Step S01). Next, basedon the collected communication quality information, the schedule fixingunit 33 for TXOP granting determines the schedule information for TXOPgranting that is made up of information on one or more STAs to which theAP 10 grants the TXOP or information which includes prioritization ofthe one or more STAs, and notifies the AP 10 of the determined scheduleinformation (Step S02). When the schedule information for TXOP grantingis input, the TXOP granting signal generating unit 15 of the AP 10determines a combination of one or more STAs to which the AP 10 grantsthe TXOP, and outputs the determined combination to the transmissionpreparation unit 13 (Step S03). The transmission preparation unit 13determines a transmission type of the TXOP granting signal in accordancewith the presence or absence of the downlink data, and notifies thetransmission and reception unit 12 of the determined transmission type(Step S04). If downlink data is present, the transmission and receptionunit 12 transmits the TXOP granting signal after transmitting thedownlink data. If the downlink data is not present, the transmission andreception unit 12 transmits the TXOP granting signal after acquiring theTXOP (Step S05). The STA 20 that receives the TXOP granting signaltransmits the user data on the uplink at a timing that is set by theTXOP granting signal (Step S06).

At this point, in the STA, if it is determined that throughput of theuplink data transmission which results from the acquisition of the TXOPcauses the user experience to decrease, four selection methods ofselecting the STA to which the AP grants the TXOP will be described.

(Method 1 of Selecting the TXOP Responder)

The scheduling unit 30 for TXOP granting collects information relatingto transmission-awaiting user data from the STA that is an AP 10'spartner in the uplink data transmission, request traffic information,and application information. Then, in each STA, the throughput of theuplink data transmission in the acquisition of the TXOP is comparedagainst the condition, and the TXOP responder is preferentially selectedfrom among STAs in which the throughput of the uplink data transmissionis in the poorest condition. A transmission resource that is allocatedby the OFDMA or the spatial multiplexing may be determined from a PHYrate between the AP 10 and the STA and bits of a shortest packet, andthe TXOP may be granted to a plurality of STA's partners. On thisoccasion, the TXOP responder may be selected from among the STAs thattransmit the user data on the downlink. Regardless of a transmissiondestination on the downlink, the TXOP responder may be selected.

With this control, the uplink data transmission can be prevented fromcausing the user experience to decrease due to low quality, and the TXOPthat is acquired by the AP can be preferentially granted to the STA thathas low quality of the uplink data transmission.

(Method 2 of Selecting the TXOP Responder)

The AP 10 measures the frame error rate of the uplink that results fromthe STA acquiring the TXOP, and based on a result of the measurement,the scheduling unit 30 for TXOP granting preferentially selects the STAthat has a high frame error rate, as the TXOP responder. Accordingly,from the frame error rate of the uplink, it can be simply detected thatthe uplink is in a poor condition, and the TXOP that is acquired by theAP can be preferentially granted to the STA that has the low quality ofthe uplink data transmission.

(Method 3 of Selecting the TXOP Responder)

The scheduling unit 30 for TXOP granting evaluates the communicationquality of the uplink and the downlink that results from the STAacquiring the TXOP, for example, the throughput or the frame error rate.As the communication quality of the uplink that results from theacquisition of the TXOP is poorer by comparison with the communicationquality of the downlink that results from the acquisition of the TXOP,the scheduling unit 30 for TXOP granting more preferentially makes aselection as the TXOP responder.

With this control, the comparison of the quality of the uplink/downlinkis performed, and thus it can be more precisely detected that a specificSTA is in a condition where the TXOP is difficult to acquire due to ahidden terminal/exposed terminal problem, than in the selection method2, and the TXOP that is acquired by the AP can be preferentially grantedto the STA that has the low quality of the uplink data transmission.

(Method 4 of Selecting the TXOP Responder)

The scheduling unit 30 for TXOP granting collects pieces of informationthat have high correlation with the communication quality, such aspositional information on the STA or average received signal strengthinformation from the STA, and information on the time at which thecommunication is performed, and selects the TXOP responder based onmachine learning or an empirical assumption, from information other thanthe communication quality and the performance of the TXOP granting sofar. For example, throughput characteristics of the uplink and thedownlink, which relates to location information on the STA, are storedin the communication quality determining unit 32 of the scheduling unit30 for TXOP granting, and thus the STA that is assumed to have the lowcommunication quality of the uplink or the downlink can be determinedusing the location information, or if the location information on theSTA and time information satisfy a fixed condition, it can be determinedthat the communication quality of the uplink or the downlink is low. Inaddition, the communication quality may be determined from informationother than direct communication, such as a function of, ownerinformation on, and application use information on the STA, and the TXOPgrant schedule may be determined.

With the methods of selecting the TXOP responder, which are describedabove, based on statistical information or the empirical assumption, theTXOP that is acquired by the AP can be preferentially granted to the STAthat has the low quality of the uplink data transmission, and the userexperience can be prevented from decreasing because of the low qualityof the uplink data transmission.

Furthermore, based on the methods of selecting the TXOP responder, whichare described above, weight to the STA or a ratio of resources on whichthe TXOP granting is performed may be determined, and a probability ofgranting TXOP to a plurality of STAs or a method of allocating afrequency or time resource block may be as the schedule information forTXOP granting.

Furthermore, the schedule fixing unit 33 for TXOP granting may acquirewireless environment information on the vicinity of the AP 10, and maylimit the number of STAs that grants the TXOP or the frequency withwhich the TXOP is granted, depending on the number of or communicationstates of unmanaged APs and unmanaged STAs in the vicinity of the AP 10.

(Downlink TXOP Granting)

Downlink TXOP granting will be described in detail.

If the AP itself acquires the TXOP in the downlink, a probability that asignal of the neighboring BSS will be always detected and thus the TXOPwill be able to be acquired is considerably low, a collision with asignal that is transmitted from a node which is a hidden terminal fromthe standpoint of the AP in the STA on the reception side is experiencedand thus the communication is not successful, and so forth. Thus, thescheduling unit 30 for TXOP granting selects a STA that hascommunication quality of the downlink that results from the acquisitionof the TXOP, which causes user experience to decrease, and sets theselected STA to be a “TXOP holder” which grants the TXOP to the AP 10.

In an example in (b) of FIG. 1, when the AP 10 is set to acquire theTXOP on the downlink, a probability that the TXOP will be acquired bythe communication over the BSS a decreases. Furthermore, although the AP10 acquires the TXOP and performs the transmission to the STA 20-1,because the communication over the BSS b that is in a hidden-terminalrelation cannot be detected, a packet collision in the STA 20-1 occursand thus a probability that the communication will fail increases. TheSTA 20-1 as in such a case is selected and the TXOP that is acquired bythe STA is granted to the AP 10, and thus the user experience on thedownlink can be improved.

FIG. 7 illustrates an example of a processing procedure for the downlinkdata transmission that results from the STA granting the TXOP.

In FIG. 7, the communication quality determining unit 32 of thescheduling unit 30 for TXOP granting collects communication qualityinformation on the downlink or the uplink/downlink (Step S11). Next,based on the collected communication quality information, the schedulefixing unit 33 for TXOP granting determines the schedule information forTXOP granting that is made up of information on one or more STAs towhich the AP 10 grants the TXOP or information which includes TXORacquisition conditions of the one or more STAs, and notifies thecorresponding STA 20 of the determined schedule information through theAP 10 (Step S12). When the schedule information for TXOP granting isinput, the TXOP granting signal generating unit 25 of the STA 20 outputsthe information that the TXOP is granted to the AP 10 and the TXORacquisition condition to the transmission preparation unit 23 (StepS13). The transmission preparation unit 23 determines a transmissiontype of the TXOP granting signal in accordance with the presence orabsence of the uplink data, and notifies the transmission and receptionunit 22 of the determined transmission type (Step S14). If uplink datais present, the transmission and reception unit 22 transmits the TXORgranting signal after transmitting the uplink data. If the uplink datais not present, the transmission and reception unit 12 transmits theTXOR granting signal after acquiring the TXOP (Step S15). The STA 10that receives the TXOR granting signal transmits the downlink data at atiming that is set by the TXOR granting signal (Step S16).

At this point, in the AP, if it is determined that throughput of thedownlink data transmission that results from the acquisition of the TXOPcauses the user experience to decrease, or that the communicationquality that results from the acquisition of the TXOP for the uplinkdata transmission is high, three methods of selecting the STA thatgrants the TXOP to the AP will be described.

(Method 1 of Selecting the TXOP Holder)

The scheduling unit 30 for TXOP granting collects information relatingto transmission-awaiting user data for the STA that is an AP 10'spartner in the downlink data transmission, the request trafficinformation, and the application information. Then, in each STA, thethroughput of the downlink data transmission that results from theacquisition of the TXOP from the AP is compared against the condition,and the TXOP holder is preferentially selected from among STAs in whichthe throughput of the downlink data transmission is in the poorestcondition. The transmission resource that is allocated by the OFDMA orthe spatial multiplexing may be determined from the PHY rate between theAP 10 and the STA and bits of a transmission packet, and the downlinkdata transmission from a plurality of STA's may be performed.

(Method 2 of Selecting the TXOP Holder)

The AP 10 measures the frame error rate of the downlink that resultsfrom the AP acquiring the TXOP, and based on a result of themeasurement, the scheduling unit 30 for TXOP granting preferentiallyselects the STA that has a high frame error rate, as the TXOP holder.

(Method 3 of Selecting the TXOP Holder)

The scheduling unit 30 for TXOP granting evaluates the communicationquality (the throughput or the frame error rate) of the uplink and thedownlink that results from the acquisition of the TXOP. As thecommunication quality of the downlink that results from the acquisitionof the TXOP is poorer by comparison with the communication quality ofthe uplink that results from the acquisition of the TXOP, the schedulingunit 30 for TXOP granting more preferentially makes a selection as theTXOP holder.

With the methods of selecting the TXOP holder, which are describedabove, based on the statistical information or the empirical assumption,the TXOP can be preferentially granted to the STA that has the lowquality of the downlink data transmission, and the user experience canbe prevented from decreasing because of the low quality of the downlinkdata transmission.

Furthermore, based on the method of selecting the TXOP holder, which aredescribed above, the frequency with which the STA grants the TXOP, thecondition that the STA grants the TXOP, or the throughput when the STAgrants the TXOP can be designated by the schedule information for TXOPgranting. For example, a contention window size of back-off may bedesignated for the acquisition of the TXOP for the TXOP granting, atiming at which the TXOP is started or a timing at which the TXOP isended may be designated, or the granting of the TXOP may be controlledin such a manner that constant throughput is satisfied, usingcommunication quality information on other external communication units,positional information on the STA, information on the function of theSTA, information on the remaining amount of battery in the STA, or thelike, as a condition for performing the TXOP granting.

EXPLANATION OF REFERENCE NUMERALS

10 WIRELESS ACCESS POINT (AP)

11 ANTENNA

12 TRANSMISSION AND RECEPTION UNIT

13 TRANSMISSION PREPARATION UNIT

14 NETWORK COMMUNICATION UNIT

15 TXOP GRANTING SIGNAL GENERATING UNIT

20 WIRELESS STATION (STA)

21 ANTENNA

22 TRANSMISSION AND RECEPTION UNIT

23 TRANSMISSION PREPARATION UNIT

24 NETWORK COMMUNICATION UNIT

25 TXOP GRANTING SIGNAL GENERATING UNIT

30 SCHEDULING UNIT FOR TXOP GRANTING

31 NETWORK COMMUNICATION UNIT

32 COMMUNICATION QUALITY DETERMINING UNIT

33 SCHEDULE FIXING UNIT FOR TXOP GRANTING

100 NETWORK

1. A wireless communication system in which, based on a random accessscheme, a transmission opportunity (TXOP) acquired by a wireless accesspoint is granted to a wireless station when performing uplink datatransmission from the wireless station to the wireless access point andthe TXOP acquired by the wireless station is granted to the wirelessaccess point when performing downlink data transmission from thewireless access point to the wireless station, the system comprising: ascheduling unit for TXOP granting determining one or more wirelessstations having a quality of the uplink data transmission lower than aprescribed value as TXOP responders to which the wireless access pointgrants the TXOP, and determining one or more wireless stations having aquality of the downlink data transmission lower than the prescribedvalue as TXOP holders which grant the TXOP to the wireless access point.2. The wireless communication system according to claim 1, wherein: thescheduling unit for TXOP granting is configured to determine one or moreof the TXOP responders having a parameter lower than the prescribedvalue, and to transmit a notification signal representing information onthe determination to the wireless access point, the parameter indicatesthe quality of the uplink data transmission; the wireless access pointincludes a unit transmitting a TXOP granting signal indicating a timingto perform the uplink data transmission to the TXOP responders afteracquiring the information on the determination of the TXOP responders bythe notification signal and acquiring the TXOP using the random accessscheme; and the wireless station includes a unit transmitting a dataframe on the uplink at the indicated timing when received the TXOPgranting signal.
 3. The wireless communication system according to claim2, wherein the scheduling unit for TXOP granting is configured to use,as the parameter indicating the quality of the uplink data transmission,at least one of a value indicating degradation in the quality of theuplink data transmission in contrast to the quality of the downlink datatransmission, a frame error rate of the uplink, a TXOP acquisition rateof the uplink, a ratio of throughput to one of a quality of service(QoS) condition and a traffic volume demand in the uplink, and a frameretransmission rate of the uplink.
 4. The wireless communication systemaccording to claim 2, wherein the scheduling unit for TXOP granting isconfigured to collect uplink traffic information relating to one of atraffic volume and a transmission-awaiting packet generated by thewireless station, and to determine as the TXOP responders when thequality of the uplink data transmission is lower than the prescribedvalue and a demand for uplink traffic in the wireless station is higherthan the prescribed value.
 5. The wireless communication systemaccording to claim 1, wherein; the scheduling unit for TXOP granting isconfigured to determine one or more of the TXOP holders having aparameter lower than the prescribed value, and to transmit anotification signal representing information on the determination to thewireless station, the parameter indicates the quality of the downlinkdata transmission; the wireless station includes a unit , transmitting aTXOP granting signal indicating a timing to perform the downlink datatransmission to the wireless access point after recognizing as the TXOPholders by the notification signal and acquiring the TXOP using therandom access scheme; and the wireless access point includes a unittransmitting a data frame on the downlink at the indicated timing whenreceived the TXOP granting signal.
 6. The wireless communication systemaccording to claim 5, wherein the scheduling unit for TXOP granting isconfigured to use, as the parameter indicating the quality of thedownlink data transmission, at least one of a value indicatingdegradation in the quality of the downlink data transmission in contrastto the quality of the uplink data transmission, a frame error rate ofthe downlink, a ratio of throughput to one of a quality of service (QoS)condition and a traffic volume demand in the downlink, and a frameretransmission rate of the downlink.
 7. The wireless communicationsystem according to claim 5, wherein the scheduling unit for TXOPgranting is configured to collect downlink traffic information relatingto one of a traffic volume and a transmission-awaiting packet from thewireless access point to the wireless station, and to determine as theTXOP responders when the quality of the downlink data transmission islower than the prescribed value and a demand for downlink traffic in thewireless access point is higher than the prescribed value.
 8. A wirelesscommunication method in which, based on a random access scheme, a TXOPacquired by a wireless access point is granted to a wireless stationwhen performing uplink data transmission from the wireless station tothe wireless access point, the method comprising: a step causing ascheduling unit for TXOP granting, which monitors a quality of theuplink data transmission, to determine one or more wireless stationshaving a parameter lower than a prescribed value as TXOP responders towhich the wireless access point grants the TXOP, and to transmit anotification signal representing information on the determination to thewireless access point, the parameter indicates the quality of the uplinkdata transmission; a step causing the wireless access point to transmita TXOP granting signal indicating a timing to perform the uplink datatransmission to the TXOP responders after causing to acquire theinformation on the determination of the TXOP responders by thenotification signal and to acquire the TXOP using the random accessscheme; and a step causing the wireless station to transmit a data frameon the uplink at the indicated timing when received the TXOP grantingsignal.
 9. A wireless communication method in which, based on a randomaccess scheme, a TXOP acquired by a wireless station is granted to awireless access point when performing downlink data transmission fromthe wireless access point to the wireless station, the methodcomprising: a step causing a scheduling unit for TXOP granting, whichmonitors a quality of the downlink data transmission, to determine oneor more wireless stations having a parameter lower than a prescribedvalue as TXOP holders which grant the TXOP to the wireless access point,and to transmit a notification signal representing information on thedetermination to the wireless station, the parameter indicates thequality of the downlink data transmission; a step causing the wirelessstation to transmit a TXOP granting signal indicating a timing toperform the downlink data transmission to the wireless access pointafter causing to recognize as the TXOP holders by the notificationsignal and to acquire the TXOP using the random access scheme; and astep causing the wireless access point to transmit a data frame on thedownlink at the indicated timing when received the TXOP granting signal.